with an uncertainty of 3 arcmin (radius, 90% containment, including
systematic uncertainty). The BAT light curve showed a complex
structure with a duration of at least 50 sec. The peak count rate
was ~2000 counts/sec (15-350 keV), at ~6 sec after the trigger.
The XRT began observing the field at 22:04:30.1 UT, 140.9 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source with an enhanced position: RA, Dec 286.8835,
-16.4196 which is equivalent to:

RA(J2000) = 19h 07m 32.03s
Dec(J2000) = -16d 25' 10.4"

with an uncertainty of 2.3 arcseconds (radius, 90% containment). This
location is 45 arcseconds from the BAT onboard position, within the BAT
error circle. This position may be improved as more data are received;
the latest position is available at http://www.swift.ac.uk/sper.
A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 1.41
x 10^21 cm^-2 (Willingale et al. 2013).
The initial flux in the 2.5 s image was 2.73e-09 erg cm^-2 s^-1 (0.2-10
keV).
UVOT took a finding chart exposure of 150 seconds with the White filter
starting 152 seconds after the BAT trigger. No credible afterglow candidate has
been found in the initial data products. The 2.7'x2.7' sub-image covers 100% of
the XRT error circle. Because of the density of catalogued stars, further
analysis is required to report an upper limit for any afterglow in the
sub-image. The 8'x8' region for the list of sources generated on-board covers
100% of the XRT error circle. Because of the density of catalogued stars,
further analysis is required to report an upper limit for any afterglow in the
region. No correction has been made for the expected extinction corresponding
to E(B-V) of 0.19.
Burst Advocate for this burst is P.A. Evans (pae9 AT star.le.ac.uk).
Please contact the BA by email if you require additional information
regarding Swift followup of this burst. In extremely urgent cases, after
trying the Burst Advocate, you can contact the Swift PI by phone (see
Swift TOO web site for information: http://www.swift.psu.edu/too.html.)

GCN Circular #21174
A. Pozanenko (IKI), D. Chestnov (ISON), A. Novichonok (ISON), A. Mokhnatkin
(ISON) report on behalf of larger GRB follow-up collaboration:
We observed the field of the GRB 170531B (Evans et al.,GCN 21171) with
with Zeiss-1000/Koshka telescope starting on May 31 (UT) 22:32:32, i.e. ~30
minutes after burst onset. We took several images in filter R with
exposures of 180 s. Near the border of XRT error circle (Evans et al.,GCN
21171) we detect the source in coordinates (J2000) 19:07:31.67 -16:25:09.3
with uncertainity of 0.5 arcsec in both coordinates. The source is probably
coincedent with the object reported by (Lipunov et al., GCN 21172). The
source is fading from R=19.1 at (UT) 22:32:32 down to R=19.5 at (UT)
23:30:56. Most probably the source is an afterglow of GRB 170531B.

with an uncertainty of 1.8 arcsec (radius, 90% confidence).
This position may be improved as more data are received. The latest
position can be viewed at http://www.swift.ac.uk/xrt_positions. Position
enhancement is described by Goad et al. (2007, A&A, 476, 1401) and Evans
et al. (2009, MNRAS, 397, 1177).
This circular was automatically generated, and is an official product of the
Swift-XRT team.

GCN Circular #21177
A. de Ugarte Postigo (IAA-CSIC, DARK/NBI), L. Izzo (IAA-CSIC),=20
D.A. Kann (IAA-CSIC), C.C. Thoene (IAA-CSIC), Z. Cano=20
(IAA-CSIC), J.P.U. Fynbo (DARK/NBI), D. Garcia Alvarez=20
(GTC, IAC) report on behalf of a larger collaboration:
We observed the afterglow of GRB 170531B (Evans et al. GCN=20
21171, Lipunov et al. GCN 21173, Pozanenko et al. GCN 21174)=20
using OSIRIS at the 10.4 m GTC telescope in La Palma (Spain).=20
Observation consisted in 3x900s exposures using the R1000B=20
grism, covering the range between 3700 and 7880 AA. The mean=20
epoch of the combined spectrum is 03:09:29 UT (5.12 hr after the=20
GRB onset). Additionally, g, r, i, and z-band observations were=20
obtained. We note that the coordinates of the afterglow are=20
19:07:32.09, -16:25:05.8 (J2000 +/-0.5=E2=80=9D), which places the GRB=20
approximately 5=E2=80=9D away from those of the source identified by=20
Lipunov et al. (GCN 21173) and Pozanenko et al. (GCN 21174).=20
The afterglow can be also detected in the early Swift/UVOT=20
images. In the acquisition image we measure a magnitude of=20
r_AB =3D 21.9 for the afterglow, as compared to PANSTARRS=20
catalogue field stars.
The afterglow spectrum shows continuum throughout the=20
wavelength range and several strong absorption features=20
corresponding to Lyman-alpha, SiII, OI, CII, SiIV, CIV, AlII and AlII=20
at a common redshift of 2.366, which we identify as the redshift of=20
the GRB.

GCN Circular #21178
P. Schady and T. Kruehler (both MPE) report
We observed the field of GRB 170531B (Swift trigger 755354; Evans et al.,
GCN #21171) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008,
PASP 120, 405) mounted at the 2.2 m MPG telescope at ESO La Silla
Observatory (Chile).
Observations started at 03:55 UT on 01-06-2017, 5.9 hours the GRB trigger.
They were performed at an average seeing of 1.3" and at an average airmass
of 1.3.
We detect a point source within the 1.8" enhanced Swift/XRT error circle
(Osborne et al., GCN #21175), and consistent with the afterglow position
reported by Ugarte Postigo et al. (GCN #21177).
Based on a total exposure of 54 min in g'r'i'z' and 45 min in JHK, we
estimate preliminary magnitudes (all in AB) of

Given magnitudes are calibrated against PanSTARRS and 2MASS field stars
and are not corrected for the expected Galactic foreground extinction
corresponding to a reddening of E_(B-V)=0.16 mag in the direction of the
burst (Schlafly & Finkbeiner 2011).
We acknowledge the excellent support provided by the Chilean observer at
the telescope, and the support astronomer, Sam Kim, in obtaining these
data.

If the light curve continues to decay with a power-law decay index of
1.18, the count rate at T+24 hours will be 3.3 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.1 x
10^-13 (1.6 x 10^-13) erg cm^-2 s^-1.
The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00755354.
This circular is an official product of the Swift-XRT team.

GCN Circular #21183
A. Cucchiara, D. Morris (U. of Virgin Islands), C. Guidorzi
(U. Ferrara), reports on behalf of a larger collaboration:
"On June 1.00 UT (T_0 +2.0h) we began observing the center
of the field of GRB 170531B (Evans et al. GCN 211171)
using the Las Cumbres Observatory 1m Sutherland facility.
We performed a series of 10x60s observations in R and I band
for a total of 10 minutes on sky in each filter. Seeing
conditions were poor.
We identified no optical counterpart within the center of
Swift-XRT refined position (Evans et al. GCN 21175), or at
the location of the identified counterpart (Lipunov et al.
GCN 21173, Pozanenko et al. GCN 21174, Schady et al. GCN 21178)
at the following 3-sigma limits:

R > 19.9 mag
I > 19.6 mag

These magnitude are calibrated against nearby USNO-B1 sources,
and are not corrected for Galactic extinction."

with an uncertainty of 1.5 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 96%.
The mask-weighted light curve shows a multi-peaked structure that starts
at ~T0 and ends at ~T+180 s. T90 (15-350 keV) is 164.13 +- 8.82 sec
(estimated error including systematics).
The time-averaged spectrum from T+0.55 to T+181.67 sec is best fit by a simple
power-law model. The power law index of the time-averaged spectrum is
1.95 +- 0.14. The fluence in the 15-150 keV band is 2.0 +- 0.2 x 10^-6 erg/cm2.
The 1-sec peak photon flux measured from T+42.71 sec in the 15-150 keV band
is 0.8 +- 0.2 ph/cm2/sec. All the quoted errors are at the 90% confidence
level.
The results of the batgrbproduct analysis are available at
http://gcn.gsfc.nasa.gov/notices_s/755354/BA/

with an estimated uncertainty of 0.1 arc sec. (radius, 90% confidence).
This position is 3.0 arc sec away from the enhanced XRT position
(Osborne et al. GCN Circ. 21175). Given the fading, detection
by other observatories and lack of matching source in the DSS, we find it
likely to be the GRB afterglow.
Preliminary 3-sigma magnitudes upper limits using the UVOT photometric system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the first
finding chart (FC) exposure and subsequent exposures are:

GCN Circular #21210
E. Mazaeva (IKI), A. Pozanenko (IKI), D. Chestnov (ISON), A. Novichonok
(ISON), V. Agletdinov (KIAM), I. Nikolenko (CrAO), A. Volnova (IKI),
report on behalf of larger GRB follow-up collaboration:
We report refined analysis of Zeiss-1000/CrAO observations. Results
reported below supersede GCN cir. 21174.
We observed the field of the GRB 170531B (Evans et al., GCN 21171) with
Zeiss-1000/Koshka (CrAO) telescope between May 31 (UT) 22:32:32 and
June 01 (UT) 00:58:11. We took several unfiltered images with
exposure of 180 s. The afterglow (Lipunov et al. GCN 21173;
Pozanenko et al. GCN 21174) is clearly visible in each image in first
initial images, and detected later in combined images. The finding chart
of the combined image can be found at
http://grb.rssi.ru/GRB170531B/GRB170531B_Koshka_170531_sources.png
The coordinates of the afterglow (source A in the finding chart) are
(J2000) 19:07:32.10 -16:25:06.0 with uncertainty of 0.5 arcsec in both
coordinates. The coordinates are consistent with the position reported
by de Ugarte Postigo et al., GCN 21177, Tyurina et al., GCN 21179,
and Siegel et al., GCN 21188.
Preliminary photometry of the afterglow can be found in the light curve at
http://grb.rssi.ru/GRB170531B/GRB170531B_light_curve_v2.png
The photometry is based on nearby USNO-B1.0 stars, calibrated against to
R2 magnitudes

Due to presence of sources in near vicinity to the afterglow the
photometry of wide FOV telescopes might be affected by the sources. In
particular our photometry reported in GCN 21189 is apparently biased by
the two sources C and D (see the finding chart). We report coordinates
and brightness of the nearby sources.